Ball Bearings 16101-2RS, the inner diameter is 12 mm, outer diameter is 30 mm and the width is 8mm, this is a popular size that could be used in many applications, Bearings are made of Chrome Steel and can rotate up to 15000 rpm, each bearing has 2 Rubber Seals to protect the bearing from dust or any possible contamination, also bearings are pre-lubricated with grease,